/*
 * Copyright (c) 2015-2017, ARM Limited and Contributors. All rights reserved.
 *
 * SPDX-License-Identifier: BSD-3-Clause
 */

#include <arch_helpers.h>
#include <assert.h>
#include <bakery_lock.h>
#include <cpu_data.h>
#include <platform.h>
#include <string.h>

/*
 * Functions in this file implement Bakery Algorithm for mutual exclusion with the
 * bakery lock data structures in cacheable and Normal memory.
 *
 * ARM architecture offers a family of exclusive access instructions to
 * efficiently implement mutual exclusion with hardware support. However, as
 * well as depending on external hardware, these instructions have defined
 * behavior only on certain memory types (cacheable and Normal memory in
 * particular; see ARMv8 Architecture Reference Manual section B2.10). Use cases
 * in trusted firmware are such that mutual exclusion implementation cannot
 * expect that accesses to the lock have the specific type required by the
 * architecture for these primitives to function (for example, not all
 * contenders may have address translation enabled).
 *
 * This implementation does not use mutual exclusion primitives. It expects
 * memory regions where the locks reside to be cacheable and Normal.
 *
 * Note that the ARM architecture guarantees single-copy atomicity for aligned
 * accesses regardless of status of address translation.
 */

#ifdef PLAT_PERCPU_BAKERY_LOCK_SIZE
/*
 * Verify that the platform defined value for the per-cpu space for bakery locks is
 * a multiple of the cache line size, to prevent multiple CPUs writing to the same
 * bakery lock cache line
 *
 * Using this value, if provided, rather than the linker generated value results in
 * more efficient code
 */
CASSERT((PLAT_PERCPU_BAKERY_LOCK_SIZE & (CACHE_WRITEBACK_GRANULE - 1)) == 0, \
    PLAT_PERCPU_BAKERY_LOCK_SIZE_not_cacheline_multiple);
#define PERCPU_BAKERY_LOCK_SIZE (PLAT_PERCPU_BAKERY_LOCK_SIZE)
#else
/*
 * Use the linker defined symbol which has evaluated the size reqiurement.
 * This is not as efficient as using a platform defined constant
 */
extern void *__PERCPU_BAKERY_LOCK_SIZE__;
#define PERCPU_BAKERY_LOCK_SIZE ((uintptr_t)&__PERCPU_BAKERY_LOCK_SIZE__)
#endif

#define get_bakery_info(cpu_ix, lock)    \
    (bakery_info_t *)((uintptr_t)lock + cpu_ix * PERCPU_BAKERY_LOCK_SIZE)

#define write_cache_op(addr, cached)    \
                do {    \
                    (cached ? dccvac((uintptr_t)addr) :\
                        dcivac((uintptr_t)addr));\
                        dsbish();\
                } while (0)

#define read_cache_op(addr, cached)    if (cached) \
                        dccivac((uintptr_t)addr)

/* Helper function to check if the lock is acquired */
static inline int is_lock_acquired(const bakery_info_t *my_bakery_info,
                            int is_cached)
{
    /*
     * Even though lock data is updated only by the owning cpu and
     * appropriate cache maintenance operations are performed,
     * if the previous update was done when the cpu was not participating
     * in coherency, then there is a chance that cache maintenance
     * operations were not propagated to all the caches in the system.
     * Hence do a `read_cache_op()` prior to read.
     */
    read_cache_op(my_bakery_info, is_cached);
    return !!(bakery_ticket_number(my_bakery_info->lock_data));
}

static unsigned int bakery_get_ticket(bakery_lock_t *lock,
                        unsigned int me, int is_cached)
{
    unsigned int my_ticket, their_ticket;
    unsigned int they;
    bakery_info_t *my_bakery_info, *their_bakery_info;

    /*
     * Obtain a reference to the bakery information for this cpu and ensure
     * it is not NULL.
     */
    my_bakery_info = get_bakery_info(me, lock);
    assert(my_bakery_info);

    /* Prevent recursive acquisition.*/
    assert(!is_lock_acquired(my_bakery_info, is_cached));

    /*
     * Tell other contenders that we are through the bakery doorway i.e.
     * going to allocate a ticket for this cpu.
     */
    my_ticket = 0;
    my_bakery_info->lock_data = make_bakery_data(CHOOSING_TICKET, my_ticket);

    write_cache_op(my_bakery_info, is_cached);

    /*
     * Iterate through the bakery information of each contender to allocate
     * the highest ticket number for this cpu.
     */
    for (they = 0; they < BAKERY_LOCK_MAX_CPUS; they++) {
        if (me == they)
            continue;

        /*
         * Get a reference to the other contender's bakery info and
         * ensure that a stale copy is not read.
         */
        their_bakery_info = get_bakery_info(they, lock);
        assert(their_bakery_info);

        read_cache_op(their_bakery_info, is_cached);

        /*
         * Update this cpu's ticket number if a higher ticket number is
         * seen
         */
        their_ticket = bakery_ticket_number(their_bakery_info->lock_data);
        if (their_ticket > my_ticket)
            my_ticket = their_ticket;
    }

    /*
     * Compute ticket; then signal to other contenders waiting for us to
     * finish calculating our ticket value that we're done
     */
    ++my_ticket;
    my_bakery_info->lock_data = make_bakery_data(CHOSEN_TICKET, my_ticket);

    write_cache_op(my_bakery_info, is_cached);

    return my_ticket;
}

void bakery_lock_get(bakery_lock_t *lock)
{
    unsigned int they, me, is_cached;
    unsigned int my_ticket, my_prio, their_ticket;
    bakery_info_t *their_bakery_info;
    unsigned int their_bakery_data;

    me = plat_my_core_pos();
#ifdef AARCH32
    is_cached = read_sctlr() & SCTLR_C_BIT;
#else
    is_cached = read_sctlr_el3() & SCTLR_C_BIT;
#endif

    /* Get a ticket */
    my_ticket = bakery_get_ticket(lock, me, is_cached);

    /*
     * Now that we got our ticket, compute our priority value, then compare
     * with that of others, and proceed to acquire the lock
     */
    my_prio = PRIORITY(my_ticket, me);
    for (they = 0; they < BAKERY_LOCK_MAX_CPUS; they++) {
        if (me == they)
            continue;

        /*
         * Get a reference to the other contender's bakery info and
         * ensure that a stale copy is not read.
         */
        their_bakery_info = get_bakery_info(they, lock);
        assert(their_bakery_info);

        /* Wait for the contender to get their ticket */
        do {
            read_cache_op(their_bakery_info, is_cached);
            their_bakery_data = their_bakery_info->lock_data;
        } while (bakery_is_choosing(their_bakery_data));

        /*
         * If the other party is a contender, they'll have non-zero
         * (valid) ticket value. If they do, compare priorities
         */
        their_ticket = bakery_ticket_number(their_bakery_data);
        if (their_ticket && (PRIORITY(their_ticket, they) < my_prio)) {
            /*
             * They have higher priority (lower value). Wait for
             * their ticket value to change (either release the lock
             * to have it dropped to 0; or drop and probably content
             * again for the same lock to have an even higher value)
             */
            do {
                wfe();
                read_cache_op(their_bakery_info, is_cached);
            } while (their_ticket
                == bakery_ticket_number(their_bakery_info->lock_data));
        }
    }
    /* Lock acquired */
}

void bakery_lock_release(bakery_lock_t *lock)
{
    bakery_info_t *my_bakery_info;
#ifdef AARCH32
    unsigned int is_cached = read_sctlr() & SCTLR_C_BIT;
#else
    unsigned int is_cached = read_sctlr_el3() & SCTLR_C_BIT;
#endif

    my_bakery_info = get_bakery_info(plat_my_core_pos(), lock);

    assert(is_lock_acquired(my_bakery_info, is_cached));

    my_bakery_info->lock_data = 0;
    write_cache_op(my_bakery_info, is_cached);
    sev();
}
